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Understanding Magma Evolution at Campi Flegrei (Campania, Italy) Volcanic 1
Complex Using Melt Inclusions and Phase Equilibria 2
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Cannatelli C.a,*, Spera F.J.a, Fedele L.b, De Vivo B.c 4
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a Department of Earth Science and Institute for Crustal Studies, University of California, Santa 6
Barbara, CA 93106 USA 7
b Department of Geosciences Virginia Tech, 4044 Derring Hall, Blacksburg, VA 24061 USA 8
c Dipartimento di Scienze della Terra, Università di Napoli Federico II, 80134 Napoli, Italy 9
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* Corresponding author: Tel. 1-805-893-8231, Fax: 1-805-893-8649 11
E-mail addresses: [email protected] (C. Cannatelli), [email protected] (F.J. Spera), 12
[email protected] (L. Fedele), [email protected] (B. De Vivo) 13
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Abstract 25
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The magmatic evolution of Campi Flegrei (Italy) has been investigated using thermodynamic 27
modeling (MELTS) and data from melt inclusions in phenocrysts from the Fondo Riccio (9.5 ka) 28
and Minopoli 1 (11.1 ka) eruptions. Thermodynamic modeling enables one to test possible 29
petrogenetic scenarios by providing constraints on eruptive mechanisms. Assuming isobaric 30
fractional crystallization is the dominant process, major element evolution and corresponding 31
changes in the physical and thermodynamic properties of the magma bodies from which Fondo 32
Riccio and Minopoli 1 magmas were erupted can be tracked. Using olivine hosted melt inclusions 33
as representative of parental melt from which the eruptive products of Fondo Riccio and Minopoli 1 34
were derived, the physical conditions (pressure, temperature, oxygen buffer, dissolved water 35
content of melt, melt density, compressibility and viscosity) and crystallization path have been 36
modeled. Results are compared to observed crystal, whole rock and homogenized melt inclusion 37
(hosted in olivine and clinopyroxene) compositions, to evaluate the extent phase equilibria 38
modeling can reproduce observations under the imposed conditions. The simulations show that 39
Fondo Riccio parental magma was likely trachyandesitic, approximated by the composition of MI’s 40
in olivine (SiO2 = 46.8%, MgO = 9.45 %), which evolved mainly through fractional crystallization 41
at low pressure (P ≈ 0.2 GPa, ≈ 6 km depth), along the QFM±1 oxygen buffer with an initial 42
dissolved H2O content of circa 3 wt%. Minopoli 1 parental magma was also trachyandesitic and it is 43
approximated by the chemistry of MIs in olivine (SiO2 = 47.8%, MgO = 9.37%) as well. The 44
estimated mean pressure of crystallization of P ≈ 0.3 GPa (≈ 9 km depth) and oxygen fugacity 45
(along QFM+1) is similar to that of FR although its initial H2O content of ~ 2 wt% is slightly less 46
than that of Fondo Riccio. Phase equilibria modeling also suggest that mafic parental magma 47
crystallized by about 50% to generate the more evolved (erupted) compositions. Melt inclusions in 48
olivine phenocrysts, the first phenocryst predicted to crystallize, evidently represent fossil remnants 49
of the parental magma. Melt inclusions within later formed clinopyroxene phenocrysts do not 50
appear to represent equilibrium liquids trapped along the liquid line of descent suggesting that 51
reaction between trapped melt and clinopyroxene may be important. The relationship between 52
fraction melt and temperature reveals the presence of a pseudo-invariant temperature, Tinv= 880° for 53
FR. The fraction of melt decreases abruptly at Tinv due to simultaneous crystallization of alkali 54
feldspar and plagioclase. The melt density, viscosity and dissolved water content change abruptly in 55
a very small temperature interval around Tinv. At this temperature, the volume fraction of exsolved 56
H2O present within magma changes from less than 10% to more than 60 vol % and exceeds the 57
fragmentation limit of circa 60 vol% for Fondo Riccio differentiated parent melt. In the case of 58
Minopoli 1, simulations do not point to abrupt ‘invariant temperature behavior’ but instead melt 59
fraction (fm) varies from 0.5 to 0.2 in a temperature span of 90°C (around 990°C), due to the 60
crystallization of alkali feldspars, plagioclase and biotite. The different eruptive style of Fondo 61
Riccio and Minopoli 1 may be related to their different volatile contents (especially water) in 62
agreement with H2O contents measured by EMPA and SIMS for both eruptions. Fondo Riccio’s 63
explosive eruption occurred more centrally in the CF region, whereas Minopoli 1 eruption occurred 64
along a fissure influenced by the regional fault system in the northern portion of the CF complex 65
graben-caldera. A simple thermal model based on variation of enthalpy of the system along the 66
liquid line of descent allowed us to estimate the duration of the differentiation event, suggesting a 67
timescale for Fondo Riccio of 6.5 ± 3.5 kyr and for Minopoli of 2.5±1.5 kyr from the beginning of 68
fractionation until eruption. 69
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1.1 Introduction 73
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Campi Flegrei (Italy) is the most active magmatic system in the Mediterranean region and has 75
exhibited predominantly explosive volcanic activity for more than 300,000 years (Pappalardo et al., 76
2002). The area is well known for its intense hydrothermal activity, frequent earthquakes and long 77
history of bradyseism including the recent episodes in 1969-1972 and 1982-1984. The city of 78
Naples and surroundings, with ~4 million inhabitants, represents one of the most densely populated 79
and volcanically active areas on Earth. The origins of Campi Flegrei’s explosive volcanism have 80
been the focus of intense research for hundreds of years and is still debated today (Di Girolamo et 81
al, 1984; Rosi and Sbrana, 1987; Barberi et al., 1991; Pappalardo et al., 1999; De Vivo et al., 2001; 82
Rolandi et al., 2003; De Astis et al., 2004; Marianelli et al. 2006; Bodnar at el., 2007; Di Vito et al., 83
2008; Lima et al., 2009). 84
Explosive volcanic eruptions constitute a challenge for volcanologists because of their 85
unpredictability; identification of the parameters determining the style of an eruption is of 86
fundamental importance in efforts to understand how explosive volcanoes work. Development of 87
models for volcanic eruption forecasting require information on the pre-eruptive chemical and 88
physical characteristics of the magmatic system (Anderson et al., 2000; Webster et al., 2001; 89
Roggensack et al., 2001; De Vivo et al., 2005; Metrich and Wallace 2008; Moore 2008). In 90
particular the pre-eruptive composition of the magma before the eruption, including its dissolved 91
volatile content, is of critical importance because composition exerts a fundamental control of 92
magma properties and hence the style of eruptive events (Anderson, 1976; Burnham, 1979). The 93
exsolution and expansion of volatiles (especially H2O) provides the mechanical energy that drives 94
explosive volcanic eruptions. The physical properties of magmas, such as density and viscosity, 95
(Lange 1994; Ochs and Lange, 1999; Spera et al, 2000) along with the pre-eruptive phase equilibria 96
(Moore and Carmichael, 1998) are strongly influenced by the dissolution of volatiles in magma and 97
affect the volcanic style of a magmatic system (Sparks et al., 1994). 98
Melt inclusions (MI) are a powerful tool to investigate the pre-eruptive magma composition 99
since they potentially retain the pristine composition of the magma at the time of trapping (Roedder 100
1984). The original volatile content of magma can be estimated by analyzing melt inclusions (MI) 101
contained in phenocrysts (Anderson, 1974; Clocchiatti, 1975; Roedder, 1979; Belkin et al., 1985; 102
Sobolev, 1990; Lowenstern, 1994; Anderson, 2003; De Vivo and Bodnar, 2003; Wallace, 2005). 103
Moreover, MI provide information concerning crystallization and mixing histories of magmas and 104
also the conditions of primary melt generation and extraction (Roedder, 1984; Carroll and 105
Holloway, 1994; Lowenstern, 1994; Sobolev, 1996; Danyushevsky et al., 2000; Frezzotti, 2001). 106
In the present work we examine the origin of magma erupted during the Fondo Riccio, FR 107
(9.5 ka) and Minopoli 1, M1 (10.3 ka) events by deriving constraints imposed from phase equilibria 108
embodied in the MELTS thermodynamic model (Ghiorso and Sack, 1995), from phenocryst and 109
glass compositions and from an analysis of MI’s. Using olivine hosted melt inclusions as 110
representative of parental melt that generated the eruptive products of Fondo Riccio and Minopoli 111
1, estimates of the pressure, temperature, oxygen buffer, density and viscosity can be made 112
assuming fractional crystallization was the dominant process of geochemical evolution. Similarly to 113
the Campanian Ignimbrite study (Fowler et al., 2007), an important aspect of our result is the 114
identification of a pseudo-invariant temperature (Tinv) along the liquid line of descent in the case of 115
the FR system. At this temperature, the melt fraction dramatically increases isothermally and the 116
volume fraction of the coexisting fluid phase rises accordingly. Additional melt properties such as 117
density and melt viscosity also undergo rapid variations. The net effect of these changes is to drive 118
the system towards dynamic instability, the culmination of which leads to eruption (Fowler and 119
Spera, 2008). While for Fondo Riccio, Tinv= 880°C, the temperature interval for significant change 120
for the Minopoli 1 eruption is much broader, around 70 K and may correlate with the less explosive 121
nature of the Minopoli 1 eruption. A simple thermal model based on variation of enthalpy of the 122
system along the liquid line of descent that affords an estimate the timescale between the start of 123
significant crystallization and the time of eruption is also presented. We apply the model to explain 124
the timescale and the mechanisms of the magmatic systems for Fondo Riccio and Minopoli 1. 125
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2.1 Volcanological background 127
Campi Flegrei Volcanic District (CFVD) is a large volcanic complex (~ 200 km2) located west of 128
the city of Naples, Italy (Fig.1). Multiple eruptions have occurred in this area in the last 300 ka 129
(Pappalardo et al., 2002), as well as intense hydrothermal activity, bradyseismic events and frequent 130
earthquakes. Two major eruptions in the CFVD include the 39 ka Campanian Ignimbrite (CI) ((Rosi 131
and Sbrana, 1987; Orsi et al., 1996) and the 15 ka Neapolitan Yellow Tuff (NYT) (Deino et al., 132
2004). De Vivo et al. (2001) and Rolandi et al. (2003) suggested that most eruptive centers align 133
along fractures activated along the neotectonic Apennine fault system parallel to the Tyrrhenian 134
coastline. They argue that eruptions from >300 ka to 19 ka are not confined to a unique volcanic 135
center or isolated vent system in Campi Flegrei as suggested by Rosi and Sbrana, 1987 and Orsi et 136
al., 1996. Rolandi et al., (2003) argued that only the Neapolitan Yellow Tuff (NYT) (15 ka, Deino et 137
al., 2004) erupted from vents within Campi Flegrei, whereas the CI (39 ka, DeVivo et al., 2001) has 138
a much wider source and dispersal area. 139
According to Pappalardo et al. (2002), the interval between the CI and NYT eruptions is 140
characterized by a large number of significantly smaller magnitude volcanic events. Since the NYT 141
eruption, margins of the region have been the site of at least 65 eruptions, divided in three periods of 142
activity. Eruptions were separated by quiescent periods marked by two widespread paleosols (Di 143
Vito et al., 1999). The last eruption in 1538 A.D. formed the Monte Nuovo cone (Di Vito et al., 144
1987) after 3.4 ka of dormancy. 145
In this paper we analyze the Fondo Riccio (FR) and Minopoli 1 (Mi1) eruptive products in 146
an effort to deduce their petrogenesis. The Fondo Riccio eruption was explosive with a strombolian 147
character and occurred at 10.3 - 9.5 kyr (D’Antonio et al., 1999) from an eruptive centre on the 148
western side of the Gauro volcano, near the centre of the Phlegrean caldera (Fig 1). The eruptive 149
deposits are limited to the vent area and lie above the Paleosol A and below the Montagna Spaccata 150
Tephra. The eruptive products consist of fallout deposits composed of very coarse scoria beds with 151
subordinate coarse ash beds (Di Vito et al., 1999). 152
According to Di Vito et al. (1999) the earlier Minopoli 1 eruption, occurred 10.3 - 11.1 ka, and 153
was strombolian with subordinate phreatomagmatic phases, while Di Girolamo et al. (1984), based 154
on the degree of dispersal of Minopoli 1’s products, define this eruption as sub-Plinian. The 155
deposits are limited to the vent area formed by scoriae horizons with a composition varying from 156
latitic to alkali-trachytic. The eruptive products are composed of alternating pumice lapilli fallout 157
and mainly massive ash fallout beds and subordinately cross laminated ash surge beds, rich in 158
accretionary lapilli (Di Vito et al., 1999). The Minopoli 1 eruption has a stronger phreatomagmatic 159
component than the closely related FR eruption based on the observed volcanic stratigraphy. 160
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3.1 Sample description and analytical techniques 162
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The locations of the samples utilized in this study are indicated in Figure 1. Here we give 164
petrographic and mineralogical descriptions of the samples and describe the methods used to 165
perform the analysis. 166
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3.1.1 Petrography and chemical composition of Fondo Riccio 168
For Fondo Riccio, CF-FR-C1 was collected at the top of the stratigraphic column and is a 169
well-vesciculated scoriae containing less than 20% by volume of phenocrysts. The phenocrysts 170
include olivine, clinopyroxene, spinel (magnetite), biotite, alkali feldspar and plagioclase. Biotite 171
occurs as large crystals (typical size ~ 2-3 mm), while apatite phenocrysts occur as small ( ~ 0.1 172
mm) acicular needles. Clinopyroxenes and feldspars commonly exhibit intergrowth textures, 173
suggesting cotectic crystallization. Olivine, clinopyroxene and plagioclase contain recrystallized 174
melt inclusions (MI), while alkali feldspar phenocrysts contain apatite inclusions. Sample, CF-FR-175
C2, is a bomb, relatively unvesciculated, containing olivine, clinopyroxene, apatite, spinel, biotite, 176
alkali feldspar and plagioclase. Olivine, clinopyroxene and alkali feldspar phenocrysts contain 177
recrystallized MI’s. Petrochemically, both samples are porphyritic latite with less than 20% 178
phenocrysts, with clinopyroxene and plagioclase often found in glomeroporphyritic clots; 179
clinopyroxene and plagioclase also occur as microlites in the groundmass. 180
In the FR samples, olivine phenocrysts range between Fo 84 and 87, and pyroxene lies in the 181
diopside-salite field on the pyroxene quadrilateral, with Wo 44 -48 and Fs 5-19. Based on 182
microprobe analyses, alkali feldspars in Fondo Riccio present a unimodal distribution with Or 183
component of ~ 79 to 88. Plagioclase crystals are zoned with An component ranging from ~ 72 to 184
98. 185
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3.1.2 Petrography and chemical composition of Minopoli 1 187
For Minopoli 1, CF-MI1-C1 was collected in the Casalesio area (Fig 1), and corresponds to 188
the base of the deposit. The sample is greyish-black scoriae, of trachybasalt composition containing 189
~ 20% phenocrysts of olivine, clinopyroxene, plagioclase, alkali feldspar, spinel (magnetite), apatite 190
and biotite. Olivine phenocrysts are weakly to unzoned with average Fo content ~ 78, while 191
pyroxenes present Wo values between 47 and 49 and Fs between 8 and 19. Based on microprobe 192
analyses, alkali feldspars in Minopoli 1 present a bimodal distribution of Or values which ranges 193
from ~ 50 to 80. Alkali feldspars exhibit zonation, with higher Or cores. Plagioclase crystals are 194
highly zoned presenting a bimodal distribution with a range from ~ 46 to 87 and peaks at 53 and 83. 195
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3.1.3 Melt Inclusions description 197
The MI’s present in both FR and M1 are generally devitrified and partially recrystallized, 198
present a bubble (shrinkage ± exsolution of volatiles) and daughter minerals (generally apatite and 199
oxides). MI’s generally have elongated ellipsoidal shapes and range from 30 to 80 µm (most 200
between 20 and 50 µm). In order to be analyzed, MI’s needed to be re-heated to a homogenous 201
glass. Detailed descriptions of melt inclusion reheating procedures, sample preparation and 202
analytical methods are elsewhere (Cannatelli et al., 2007 and reference therein). 203
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3.1.4 Analytical methods 206
Major and minor elements analyses of phenocrysts were performed in the Department of 207
Earth Science at University of California, Santa Barbara using a Cameca SX-50 electron 208
microprobe equipped with five wavelength dispersive spectrometers. Phenocrysts analyses were 209
performed using a 1µm focused beam at 15 keV accelerating voltage and a beam current of 15nA. 210
Uncertainty of analyses was in the order of 1% for most elements. Quantitative electron microprobe 211
analyses (EMPA) on phenocrysts and MIs were performed at Virginia Tech and at University of 212
Rome “La Sapienza” (IGAG-CNR, Rome, Italy) on a Cameca SX-50 equipped with four 213
wavelength dispersive spectrometers. The analytical scheme for MIs was chosen for major/minor 214
oxide analyses. Analysis of SiO2, TiO2, Al2O3, FeO, MnO2, MgO, CaO, Na2O, K2O, NiO, Cr2O3, 215
P2O5, and Cl, S and F and standardization were preformed using silicate, oxide, phosphate and glass 216
standards, and the data were corrected with the PAP method, developed by Pichou and Pouchoir 217
(1985), using vendor supplied software. Analyses were performed at 15 kV, using a current of 20 218
nA with a defocused beam diameter of 10 µm and counting time 10 seconds, as recommended by 219
Morgan and London (1996). Relative one-sigma precision is estimated to be 1 to 2 % for major 220
elements and 5 to 10 % for minor elements. In each analytical run, alkalis were counted first, and no 221
correction has been made for Na loss. Test runs made prior to the beginning of the analysis on 222
synthetic and natural glass standards of known composition showed no significant alkali migration 223
under the specified analytical conditions. 224
Selected MI’s were analyzed for H (reported as H2O), light and rare earth elements by 225
Secondary Ion mass Spectrometry (SIMS) at the Woods Hole Oceanographic Institution, using 226
techniques detailed by Shimizu and Hart (1982) and Webster et al. (1996). Accelerating potential 227
was 10 kV and beam current was 1-2 nA. Precision and accuracy were monitored with NBS 228
(National Bureau of Standards) reference glasses NBS 610. Results on the NBS glasses are similar 229
and within 5% of the accepted values; H2O concentrations are reproducible to + 0.3 to 0.4 wt% and 230
trace elements to 5 to 15% (for more details see Webster et al., 2001). 231
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4.1 Phase equilibria modeling 233
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4.1.1 Procedures to select the parental melt composition 235
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Phase equilibria modeling has been carried out using the software MELTS, a thermodynamic 237
model of crystal-liquid equilibria. The MELTS algorithm is based on classical equilibrium 238
thermodynamics and has been object of extensive reviews in the past years (Ghiorso and Sack, 239
1995, Asimow and Ghiorso, 1998). The use of MELTS to reconstruct the crystallization path of a 240
magma requires specification of initial conditions, including 1) the initial state of the system 241
(parental melt composition including H2O content, starting temperature and pressure, and oxygen 242
fugacity) and 2) constraints under which the magmatic evolution proceeds (open or closed system, 243
fractional or equilibrium crystallization, minimization of appropriate thermodynamic potential 244
based on imposed constraints). In this work we investigate isobaric crystallization scenarios and 245
explore both equilibrium and fractional crystallization scenarios. By a long shot, fractional 246
crystallization generates results closer to observations (see below). 247
The search of parental melt composition starts with the assumption that MI’s within 248
phenocryst phases can be related to a unique parental melt during cotectic (olivine +clinopyroxene) 249
crystallization. The graphical method developed by Watson (1976) is used to test the hypothesis 250
that MI’s are primary or nearly so. MI’s composition(s) of interest are further culled by selecting 251
ones that exhibit the lowest concentrations of incompatible trace elements and highest MgO 252
contents as input for the phase equilibria calculations. 253
In the case of Fondo Riccio, 7 MI’s were selected, hosted in olivine and pyroxene and have 254
been plotted on a CaO-MgO-Al2O3 coordinates, as described by Watson (1976). The intersection I 255
(Figure 2a) of olivine and clinopyroxene fractionation lines is in the field occupied by FR-C1-o6 256
M1, a melt inclusions hosted in olivine O6. This melt inclusion represents the predicted 257
composition of the melt at the cotectic point, where olivine and clinopyroxene crystallize 258
simultaneously, so it is reasonable to hypothesize that the Parental Melt (PM) composition should 259
be more primitive than FR-C1-o6 M1. The MIs FR-C1-o2 M1 (9.45 wt% MgO), and FR-C1-o1 M1 260
(8.05 wt % MgO) possess high MgO contents and the lowest concentration of incompatible trace 261
elements and are consequently considered the best candidates to represent PM. We carried out 262
phase equilibria calculations using FR-C1-o1 M1 (not shown) and FR-C1-o2 M1 and differences 263
were small; based on this we decided to select the one with the highest MgO content. 264
In the case of Minopoli 1, by applying the Watson graphical method we found that Mi1-C1-P8 265
M1, a MI hosted in the clinopyroxene P8 (fig 2b) represents the composition of the melt at the 266
cotectic point. We selected the parental melt composition choosing the MI with the highest MgO 267
content and lowest incompatible trace element concentrations as an approximation to the PM. The 268
MI that best fit the criteria and was closest to Mi1-C1-P8 M1 in Fig. 2b was hosted in olivine o5 269
with a MgO content of 9.37 wt%, and values of Ce, and Nd of 69 and 61ppm. It is probable that 270
MI’s in olivine can undergo some re-equilibration with the host (Danyushevsky and co-workers, 271
find reference; Kress and Ghiorso, 2004). However in our case the MELTS results agree very well 272
with the compositions for the MI’s in olivine for both FR amd Mi1 samples. Our interpretation of 273
these relations is that that post entrapment changes for these MI’s are small to negligible. We 274
conclude that the method espoused 35 years ago by Watson is indeed useful and that by careful use 275
of MI’s one can in this circumstances estimate the parental melt composition reasonably well. 276
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4.1.2 Phase equilibria constraints 278
To reconstruct the magmatic evolution the initial state of the system, devolatilized PM 279
composition, dissolved H2O content of PM, initial temperature, pressure, and oxygen buffer are 280
specified. Here we present results of closed system isobaric fractional crystallization where the 281
Gibbs energy is the appropriate thermodynamic potential to be minimized. These runs clearly show 282
the effects of varying pressure, fO2 and the initial water content of the parental melt on the liquid 283
line of descent and on the composition and abundance of all crystalline phases and the temperature 284
at which melt becomes water saturated. After setting P, fO2 and dissolved H2O content, we compare 285
predicted phase and melt compositions to those observed in order to determine the range of physical 286
conditions leading up to eruption for FR and M1. We selected the “best case” based on 287
correspondence between mineralogical and geochemical data and the phase equilibria calculations. 288
Calculations were rejected when the deviation between observation and model was deemed too 289
large. This is a judgment that will naturally vary from petrologist to another; there is no ab initio 290
method to judge ‘closeness’, although an experienced petrologist will be able to spot a poor 291
solution, one that provides no new insight into the petrogenesis of the system. One must keep in 292
mind the assumptions of the method and the realities of Nature. For example, the computation 293
assumes perfect fractional crystallization. However, in situations where crystals are removed from 294
liquid by some physical process such as crystal settling or liquid filter pressing, there will always be 295
some reaction between earlier formed crystals and ambient liquid. Similarly, the calculation 296
assumes there is a single parental composition from which all differentiated liquids develop. It is 297
easy to imagine that compositional heterogeneities would be present a priori even if convective 298
mixing was reasonably efficient. Additionally, the calculation assumes that crystallization is 299
isobaric, exactly. The approximate nature of this assumption should be clear to anyone who ever 300
mapped a pluton in rugged terrain. The point of performing phase equilibria calculations using an 301
imperfect thermodynamic model (no thermodynamic model is perfect) with constraints that are 302
obviously approximate is to evaluate the overall reasonability of the proposed scenario. If, for 303
example, crystallization is grossly polybaric, then no isobaric model will come close to reproducing 304
observed phase compositions, abundances and glass (melt) compositions. One could then perform a 305
constrained polybaric simulation and ask if that procedure produces better agreement. If 306
assimilation plays an important part of the petrogenesis, then no closed system phase equilibria 307
model will produce satisfactory correspondence to observations and one would rightfully seek to 308
explore petrogenetic models involving significant assimilation and reaction. In this study (see 309
below) we find that isobaric closed system fractional crystallization at low pressure produces results 310
that bear a close (but not perfect) correspondence to observed relations and that the implications of 311
the calculation suggest a causative link between crystallization and magma eruption (see below). 312
313
4.2 Fondo Riccio 314
The initial water content in the parental melt has been estimated starting from the values 315
obtained for MI’s by SIMS analyses. Fondo Riccio’s MI’s belong to two different populations of 316
inclusions, one with water contents ranging between 1 and 4 wt% and the other with water values 317
around 6 wt%. As starting water content we tested values ranging between 1 and 5 wt%, but from 318
petrographic observations values of H2O >3wt% were discarded because of the high water 319
saturation temperature. For example, in the case of H2O = 4wt% the temperature of water saturation 320
was 1070°C. At this temperature the system is saturated in water and crystallizing mineral phases 321
such as clinopyroxene, plagioclase and alkali feldspar should trap fluid inclusions during the 322
cooling process. There is no petrographic evidence of fluid inclusions hosted in these phases in the 323
samples studied here. In the cases of H2O < 2 wt%, each run generated a rhombohedral oxide phase 324
(ilmenite) at low melt fractions, inconsistent with the phase assemblage observed. Although not 325
shown, calculated runs with initial water content in the PM less than 2 wt% and greater than 4wt% 326
did not predict the phase assemblage observed in the Fondo Riccio. We therefore conclude that 327
initial water content in the PM around 3 wt% is the most realistic case for the Fondo Riccio eruptive 328
model. Although we acknowledge that this is a judgment, we believe it to be the best estimate based 329
on the congruence between calculation and observation. 330
The majority of the runs were made isobarically and for Fondo Riccio at P < 0.3 GPa; at 331
greater P the presence of predicted minerals such as garnet or muscovite is not compatible with the 332
FR phenocryst assemblage. To understand better the effect of a changing pressure on our system, 333
we compared MELTS generated TAS diagrams calculated at a fixed fO2 = QFM+1, QFM and P = 334
0.1, 0.15, 0.2 and 0.3 GPa. For the case of fO2 = QFM and QFM+1 we observe good agreement 335
between phase equilibria (MELTS) predictions with the Fondo Riccio’s data (see Fig. A in 336
Supplementary Material). The best case scenario of oxygen fugacity for Fondo Riccio was chosen 337
for P ≈ 0.2 GPa, corresponding to ~6 km depth, and compatible with recent studies by Zollo et al., 338
2003 suggesting that a hypothetical magma body at Campi Flegrei is about 6 km deep. 339
From petrographic investigation we found the presence of spinel (in the form of magnetite solid 340
solution) in olivine and clinopyroxene, but not in plagioclase and feldspars. We also noticed an 341
abundant presence of biotite. We compared several MELTS generated mineral distribution diagram 342
with our petrographic observations and we found that the best agreement is reached when fO2 varies 343
between QFM-1 and QFM+1. We also noticed, as expected, the strong dependence of the iron-344
bearing phases on the variation of oxygen fugacity. For example, when we consider the case of 345
Fondo Riccio with initial water content of 2wt%, an increase in the oxygen fugacity from QFM-2 to 346
QFM+2, stabilizes spinel at higher temperature, while not affecting the crystallization temperature 347
of clinopyroxenes and feldspars (see Fig. B in Supplementary Material). The stabilization of spinel 348
at higher temperatures corresponds to a decrease of FeOtot and increase of SiO2 content in the melt. 349
Our choice of best case has been mostly influenced by the spinel stabilization temperature; the 350
inconsistency between observed mineral assemblage and MELTS generated mineral distribution has 351
lead us to discard oxygen fugacity values of QFM-2, QFM-1 and QFM+2. 352
In summary, the physical conditions that produce the closest correspondence between the 353
model and observation is fractional crystallization of a parental melt of (anhydrous) composition 354
(given in Table 1 Supplementary Material) with 3wt % H2O added at pressure of 0.15 GPa and 355
oxygen fugacity around the QFM buffer. 356
357
4.3 Minopoli 1 358
Water contents of MIs from the Minopoli 1 eruptive products were measured by SIMS and 359
range from 1 to 4wt% (Cannatelli et al., 2007). The effect of varying the initial water concentration 360
in the parental melt was examined in the Minopoli case through isobaric fractional crystallization, 361
similarly to FR. Petrographic studies of Minopoli 1’s thin sections reveal the presence of large (1-2 362
mm) biotite crystals. The presence of such crystals implies initial water contents greater than 2 363
wt%. Therefore simulations obtained by setting the water content less than 2wt% were discarded, 364
regardless of oxygen fugacity and pressure values. Furthermore, in the case of H2O > 2wt% we 365
observed a lack of intersection between the MELTS generated oxides trends and the real data field 366
of Minopoli 1. In particular, values of water content greater of 3wt% were discarded for fO2 = QFM 367
≥ QFM+2 and pressure greater than 0.3 GPa, because of the predicted presence of garnet and 368
leucite, inconsistent with the observed assemblage. Values of water greater than 4wt% were 369
discarded because of the high water saturation temperature (T ~ 1080°C) which would result in the 370
presence of fluid inclusions in the phenocrysts of Minopoli 1 sample, not observed in Minopoli 1. 371
The initial water content of the parental melt for Minopoli 1 is around 2 wt% at bit lower than for 372
FR. 373
Several simulations were carried out using a fixed value of initial water content of 2-3wt%, 374
and varying the pressure and the oxygen fugacity. Many runs were discarded because of mismatch 375
between observed and predicted phases, such in the cases of fO2 > QFM or P ≤ 0.1GPa. A small 376
decrease in oxygen fugacity leads to a decrease of spinel stabilization temperature of almost 100°C 377
and a longer crystallization interval for feldspars with a consequent greater generated mass of 378
feldspars in the mineral assemblage. Comparisons among feldspars plotting model results and 379
observations on ternary diagrams (An-Ab-Or) and spinel diagrams (FeO-Fe2O3-TiO2) were 380
conducted in order to establish the best fit between observation and prediction. In general higher 381
pressures better match observed phases (Fig. C in Supplementary Material). In particular, for fO2 = 382
QFM+1 as starting oxygen fugacity value, we can observe a the good fit of generated spinel and 383
feldspars data with the observed Minopoli 1 phenocryst compositions especially for P=0.3GPa and 384
water content of 2 wt%. The best case chosen from the several Minopoli 1 simulations is 385
represented by a parental melt of (anhydrous) composition (Table 1 in Supplementary Material) at 386
pressure P ~ 0.3 GPa (6-9 km depth), water content of 2 wt % along the QFM+1 oxygen buffer. 387
388
5.1 Results 389
390
5.1.1 Fondo Riccio 391
392
We present results for Fondo Riccio for isobaric fractional crystallization of the estimated 393
parental composition. In fact, we have used a number of possible parental compositions and 394
although small differences in results are obtained, the salient features are robust. The parental melt 395
composition of FR-C1-o2 M1 with an initial water content of 3 wt% is used to generate the results 396
below. The fractional crystallization path along the QFM to QFM+1 oxygen buffer at 0.15 GPa has 397
been computed. MELTS correctly predicts the mineral phases observed. Olivine is the liquidus (T= 398
1260°C) phase, followed by clinopyroxene, magnetite, H2O, plagioclase, Alkali feldspar and biotite 399
at 1110°C, 1100°C, 1070°C and 880°C respectively. Mineral distribution, abundances and 400
temperature at which water saturates are shown in Fig. 3. It is interesting to note the abrupt change 401
in melt composition around 880°C due to a simultaneous crystallization of alkali feldspar, 402
plagioclase and biotite. As in the case of the Campanian Ignimbrite, we can define this temperature 403
as pseudo-invariant point temperature (Fowler et al., 2007). At this temperature, a major change in 404
melt fraction (fm), from 0.5 to 0.1 and melt composition occurs (Fig. 3). The properties of the melt 405
(density and shear viscosity) and of magma (density, volume fraction of bubbles, shear viscosity) 406
change dramatically around this temperature (see below). 407
The growth of alkali feldspars and plagioclase dominates the crystallization path at T ~ 880°C 408
and below. In fig.4 crystallization patterns for fO2 = QFM and QFM+1 are portrayed. 409
Concentrations of SiO2, K2O, Na2O and Al2O3 initially increase with decreasing MgO due to the 410
crystallization of olivine and continue to increase as clinopyroxene, spinel and apatite crystallize 411
(Fig. 4a-f). The increase of CaO concentration ends when melt becomes saturated in clinopyroxene 412
and then decreases slowly with cooling. FeOtot concentrations slightly decrease in the early stages 413
of crystallization and decrease abruptly when spinel joins the already fractionated phase minerals. 414
Results for QFM and QFM+1 are quite similar for Al2O3, K2O and Na2O while for SiO2, FeOtot and 415
CaO we can observe a more close approximation to the observed trends for fO2=QFM+1. 416
At T=Tinv there is a rapid change in the variation diagram trajectories of circa 2 wt% for SiO2 and 417
Al2O3, 1 wt% for K2O and 0.5 wt% for CaO and Na2O (Fig.4). For T<Tinv SiO2, CaO, Al2O3 and 418
K2O show a sudden decrease, while Na2O continues to increase as a result of feldspar fractionation. 419
These compositional changes at T=Tinv are associated to a change in the physical properties of both 420
melt and magma with significant consequences for eruption probability and dynamics. 421
As noted on Fig. 4, MI’s hosted in olivine and clinopyroxene agree well with the predicted 422
liquid line of descent making melt inclusions, especially hosted in olivine. There is a good 423
agreement between observed and simulated clinopyroxene and olivine compositions (Fig. D in 424
Supplementary Material) remembering that calculated values assume perfect fractional 425
crystallization. Alkali feldspar trends compare favourably; predicted plagioclase becomes more 426
sodic than observed values near the solidus presumably related to the breakdown of the assumption 427
of perfect fractional crystallization near the solidus. In addition, differences were noted for perfect 428
fractionation of both crystals and exsolved fluid or just solid crystallization. In general, the best 429
agreement was found for the case when both precipitated solids and exsolved H2O were removed in 430
fractional crystallization. The spinel ternary diagram also show good agreement between MELTS 431
predictions and spinel compositions from FR samples. 432
433
5.1.2 Minopoli 1 434
Based on the assumption that the most realistic parental melt has a composition of Mi1-C1-o5- 435
with water content of 2-3 wt%, we present results for calculations with oxygen buffer set at QFM+1 436
and pressure at 0.2-0.3 GPa. In Fig. 5 we can see the mineral distribution along the crystallization 437
path for the case P= 0.3 GPa, 2wt% H2O and fO2= QFM+1. 438
The liquidus phase is olivine at T= 1300°C, followed by clinopyroxene (T= 1160°C), spinel 439
(T=1070°C) and apatite (T=1020°C). At 990°C, 960°C and 900°C respectively plagioclase, biotite 440
and alkali feldspar join the mineral assemblage, dominating the crystallization path. 441
From Fig. 6a-f, concentrations of SiO2, Al2O3, K2O and Na2O increase with decreasing MgO 442
during the crystallization of olivine and then continue to increase as clinopyroxene, apatite and 443
spinel crystallize. The increase of CaO ends when clinopyroxene begins crystallization and then 444
decreases slowly with cooling. FeOtot concentrations slightly decrease in the early stages of 445
crystallization, then remain constant and decrease abruptly only when spinel begins fractionation. In 446
the case of Minopoli 1 an abrupt change in melt composition noted in Fondo Riccio is not evident; 447
instead, in a temperature span of about 80°C (around T= 990°C) there is a change of fm from 0.5 to 448
0.2, due mainly to the crystallization of feldspar. At T=990°C, there are changes in the calculated 449
oxides trends of about 3 wt% for SiO2, CaO and K2O, 2wt% for Al2O3 and 1 wt% for Na2O and 450
FeOtot . Parenthetically, this comparative behaviour shows how sensitive phase equilibria are to 451
small changes in melt starting composition and ambient conditions. This indicates that the approach 452
used here is not ‘one size fits all’ even when differences in the systems (FR and M1) are relatively 453
small. 454
For T<Tinv SiO2, CaO, FeOtot and K2O show a sudden decrease, while Na2O and Al2O3 continues 455
to increase as a result of feldspar fractionation. From Fig. 6, MIs hosted in olivine phenocrysts 456
agree with MELTS predicted liquid line of descent, while MI hosted in clinopyroxenes do not. 457
A possible scenario in this case could be a post-entrapment diffusive re-equilibration, with the 458
presence of trapped melt and the host crystal not in equilibrium during cooling (Qin et al., 1992; 459
Danyushevsky et al., 2000; Cottrell et al., 2002; Michael et al., 2002). If the cooling rate is slow, the 460
diffusive gradient in the crystal may extend to the host magma resulting in re-equilibration between 461
the melt inclusion and the magma surrounded the phenocrysts (Gaetani and Watson, 2000). If the 462
cooling rate is fast, such as in the case of scoriae or pumices, post-entrapment re-crystallization 463
could take place as well and the crystallization of the host mineral on the walls of the inclusion 464
modifies the composition of the melt inclusion between the time of entrapment and quenching. We 465
do not have any independent evidence of post entrapment re-equilibration in melt inclusions so the 466
cause of the divergence remains open. 467
Good agreement is found between observed and computed clinopyroxene, plagioclase and 468
alkali feldspar compositions (Fig. E in Supplementary Material), considering that simulated data are 469
calculated assuming a perfect fractional crystallization at a single pressure. The closed-system 470
model reproduces the range of observed phenocrysts compositions reasonably well. Although we 471
cannot rule out any involvement of assimilation, there is no indication that this process played a 472
critical petrogenetic role for either the FR or Minopoli 1 system. 473
474
5.1.3 Changes in properties at T=Tinv 475
476
Significant changes in properties with temperature of melt and magma can be observed in Fig. 477
7 and 8 respectively for Fondo Riccio and Minopoli 1. All variations in properties become more 478
significant near the invariant temperature Tinv, especially for Fondo Riccio. Fig. 7a and 8a shows 479
the variation of melt density with temperature along the liquid line of descent, where the most 480
dramatic change of physical properties for FR and Mi1 occurs at T ≤ Tinv, because the melt density 481
decreases as a result of a temperature decrease and mass fraction of the fluid phase ( fluid! ) 482
significant increase 483
The variation of dissolved water in the melt along the liquid line of descent can be observed in 484
Fig. 7b and 8b. For FR, melt saturates with respect to H2O at 1108°C at about 4 wt % H2O and 485
increases as crystallization occur and heat is extracted. At Tinv the H2O content jumps from about 486
4.5 wt% to 5 wt% H2O and has a rate of increase of 1 wt% H2O per 30°C. For Minopoli 1 the 487
saturation of melt with respect to H2O occurs at 800°C at about 8 wt%. Around the invariant 488
interval the value of dissolved water jumps from 3.55wt% to 4.33 wt% and increases with a rate of 489
1.0 wt% per 30°C. 490
The viscosity of melt as a function of temperature along the crystallization path is shown for 491
both eruptions in Fig. 7c and 8c respectively. For Fondo Riccio the variation of viscosity is similar 492
of what has been observed for the Campanian Ignimbrite (Fowler et al., 2007). Melt viscosity for 493
Fondo Riccio system present a cusped path; a rapid increases with falling of temperature between 494
Tliquidus and Tinv (due to cooling and the silica enrichment of evolved melt) and then a dramatic drop 495
for T<Tinv (due to the increasing concentration of water dissolved in melt). As we can see from fig. 496
8c, for Minopoli 1 the behaviour of the system is different from the FR trend: during cooling the 497
increase of viscosity and dissolved water content with temperature is more gradual. 498
In Fig. 7d, the volume fraction of water in the magma along the crystallization path for the 499
Fondo Riccio is depicted, where magma has been defined as a homogeneous mixture of 500
oversaturated melt plus bubbles of supercritical fluid (Fowler et al., 2007). The magma density was 501
calculated according to: 502
!
"magma =" fluid"melt
"melt# fluid + " fluid (1$# fluid ) (1) 503
where fluid! is the mass fraction of the fluid phase in the mixture, ρfluid is the density of exsolved 504
H2O and ρmelt is the density of volatile-saturated melt. At T=Tinv there is a dramatic increase in 505
volume fraction of water, from about 15% vol to 60% vol just below Tinv. The exsolution and 506
expansion of H2O provides the mechanical energy that drives explosive volcanic eruptions. 507
According to Cashman et al., (2000), a pyroclastic eruption can occur when the fluid volume 508
fraction exceeds roughly 70% by volume at which magma fragmentation occurs. 509
Our phase equilibria calculations are consistent with the following picture for Fondo Riccio. 510
Isobaric crystal fractionation of parental basaltic trachyandesitic melt initially containing about 511
3wt% H2O generates a liquid line of descent consistent with melt inclusion and phenocryst 512
compositional data. In the absence of magma decompression, the crystallization of almost 60% of 513
the original melt and the drastic increase in the volume fraction of supercritical fluid just below 514
Tinv= 880°C, leads to an abrupt increase of the volume of the system and consequent fluid 515
expulsion. This occurs at the same time that the liquid fraction of the system is rapidly decreasing. 516
At this point, a fluid cap develops at the top of the magma body producing roof hydrofracture and 517
the propagation of volatile-saturated magma–filled cracks. The resultant release of pressure during 518
decompression causes further exsolution of fluids from the melt since the solubility of volatiles 519
decreases as pressure is reduced. As the volume fraction of fluid in the magma increases, the 520
magma viscosity also decreases which in turn allows for even more rapid ascent. Via this 521
mechanism of positive feedback the system becomes unconditionally unstable and an eruption 522
ensues. 523
For Minopoli 1’s magma, the phase equilibria calculations suggest that the system was deeper 524
(~ 9 km depth) and drier (2wt% H2O) than Fondo Riccio. Unlike the case of FR, for Minopoli 1 525
simultaneous saturation of plagioclase, alkali feldspar and biotite crystallization took place in a 526
temperature span of ~90°C and not isothermally at Tinv as for FR. The smaller rate of change of 527
fraction crystallized with temperature naturally leads to less abrupt changes in the melt composition, 528
properties and physical state of the magma. A decrease in melt viscosity (from 105 to 104 Pa s), 529
coupled with a smaller change in the volume fraction of water in magma (from 0.05 to 0.2) and a 530
decrease in melt density nevertheless drove the system towards dynamical instability and acted as a 531
destabilizing eruption trigger. A prediction of this model is that the Minopoli 1 eruption was less 532
explosive than that of FR. This prediction may be tested by analysis of the volcanic stratigraphy and 533
by granulometric studies on available samples. Poor exposures make this test a difficult one to carry 534
out although one worth trying. 535
536
537
6.1 Timescale for Fondo Riccio and Minopoli 1 magma evolution 538
539
While phase equilibria modelling can constrain the thermodynamic and transport properties of 540
magmas, the evolutionary timescale cannot be determined without additional considerations. Here 541
we apply a simple thermal model in order to estimate the time interval between the start of 542
fractionation and the eruption in the context of the phase equilibria model. This model can be tested 543
using isotopic data on the various phenocryst phases, although these data do not presently exist. 544
The timescale is estimated by determining the time it takes for sufficient heat to be removed from 545
the magma in order to drive the geochemical evolution from liquidus to eruption temperature. That 546
is, it is assumed that parental melt of volume V (VFR or VMI for Fondo Riccio and Minopoli 1, 547
respectively) and density ρ loses heat at flux rate q& and that the total amount of heat that needs to 548
be removed is the difference in enthalpy (ΔH) between the initial and final states. The fraction of 549
parental melt volume (fm) that differentiates to form the FR and Mi1 melt compositions and the 550
fraction (α) of that volume that erupts to form Fondo Riccio and Minopoli 1 (respectively VEFR and 551
VEMI) are linked by the following: 552
VEFR = α fm VFR (3a) 553
VEMI = α fm VMI (3b) 554
The volume of the magma body that crystallizes can be expressed in function of surface area A and 555
a dimensionless constant K that depends on the shape of the magma reservoir, such that A = KV2/3. 556
The shape of the magma reservoir can be approximated with a cubical, disk-like or spherical 557
volume, for which 7 < K < 5. With these assumptions the timescale can be calculated as: 558
3/1
)( !!"
#$$%
&'=
m
EFRth
f
V
qK
HFR
(
)*
& (4a) 559
3/1
)( !!"
#$$%
&'=
m
EMIth
f
V
qK
HMI
(
)*
& (4b) 560
The time t since the start of fractionation for each mineral phase is 561
Htth!= (5) 562
Where H is the dimensionless enthalpy and it is function of melt fraction or temperature and is 563
defined: 564
!
H =Hliquidus "H (T )
#H (6) 565
566
Some parameters such as ! , H! and mf near the solidus are fairly constant and we choose values 567
of 2200 kg/m3, 1MJ/kg and 0.05 (for Minopoli 1) and 0.1 (for Fondo Riccio). At Campi Flegrei the 568
present day heat flow (q& ) range between 1 and 2.5 W/m2 (AGIP, 1987; Wohletz et al, 1999; De 569
Lorenzo et al., 2001), as measured at geothermal boreholes in Mofete and San Vito. The fraction (α) 570
of differentiated magma that erupted to form Fondo Riccio and Minopoli 1 eruptive fields can be 571
estimated between 0.5 and 1 (Crisp et al., 1984; White et al., 2006), which we have chosen as the 572
maximum and minimal values. The estimated DRE eruptive volume of Fondo Riccio and Minopoli 573
1 is 0.16 km3 and 0.1 km3 respectively (Di Girolamo et al., 1984) which leads to a timescale τ of 6.5 574
± 3.5 ka for Fondo Riccio and 2.5 ± 1.5 ka for Minopoli 1 (Fig. 9a-b). The values obtained for τ 575
using the simple thermal model allow us to approximate the timescale for the fractionation process 576
and to give an estimate of the age of each mineral phase. The more evolved compositions of Fondo 577
Riccio melt inclusions and eruptive products can be explained by the longer stationing of the batch 578
magma in the chamber before the eruption, allowing the melt to fractionate up to 60 vol %. 579
7. 1 Conclusions 580
The present study has been conducted with the goal to reconstruct the history of Fondo Riccio 581
and Minopoli 1 eruptions using a combination of melt inclusion data, thermodynamic and thermal 582
modelling. The simulations were carried out using MELTS and varying the initial water content, the 583
oxygen fugacity and the pressure. Both systems were assumed to evolve by fractional crystallization 584
in a closed system. Melt inclusions in olivine phenocrysts, the first phenocryst to crystallize, 585
evidently represent fossil remnants of the parental magma and were used to represent the starting 586
composition. Parental melt for Fondo Riccio has about 3 wt% H2O and evolved by isobaric 587
fractional crystallization at pressure near 0.15 GPa (equivalent to 5-6 km of depth) among QFM - 588
QFM+1 oxygen buffer. Calculated phase equilibria along the liquid line of descent show that for P 589
= 0.15 GPa, olivine is the liquidus phase (Tliq = 1260°C), followed by clinopyroxene (1110°C), 590
magnetite (1100°C), saturation of water (1070°C) plagioclase, alkali feldspar and biotite (880°C). 591
The calculated oxides trend and composition of phase mineral well agree with observed melt 592
inclusions and mineral assemblage suggesting that Fondo Riccio’s system has most likely evolved 593
by closed-system fractional crystallization. At a temperature of 880°C, the magmatic system is 594
subject to a dramatic variation in its physical properties (viscosity, density and water dissolution) as 595
biotite, plagioclase and alkali feldspars start to crystallize. At this temperature, an abrupt decrease in 596
the fraction of melt from 0.5 to 0.1 occurs. The sudden decrease of viscosity and density at this 597
pseudo invariant point temperature and the dramatic change in volume fraction of water from 0.1 to 598
0.6 is, we speculate, the ‘trigger’ mechanism for the eruption of Fondo Riccio magma. 599
Minopoli 1’s petrological evolution has been simulated by isobaric fractional crystallization. The 600
starting parental composition based on MI’s in olivine suggests a more primitive parent. The 601
system, containing 2 wt% H2O, has evolved from pressure of 0.3 GPa and oxygen fugacity values 602
around QFM+1. The crystallization sequence is represented by olivine (Tliq = 1300°C), followed by 603
clinopyroxene (T= 1160°C), spinel (T=1070°C), apatite (T=1020°C), plagioclase (T=990°C), 604
biotite (T=960°C) and alkali feldspar (T = 900°C). In the case of Minopoli 1, simulations have not 605
shown invariant temperature behaviour but only a variation of melt fraction (fm) from 0.5 to 0.1 in 606
a temperature span of 90°C (around 990°C), due to the crystallization of alkali feldspars, 607
plagioclase and biotite. A good agreement between observed and calculated mineral compositions 608
suggests that also Minopoli 1 has undergone to a fractional crystallization process even though melt 609
inclusions within later formed clinopyroxene phenocrysts do not appear to represent equilibrium 610
liquids trapped along the liquid line of descent. This suggests that reaction between trapped melt 611
and clinopyroxene may be important. The different eruptive style of Fondo Riccio and Minopoli 1 612
may be related to their different volatile contents in agreement with H2O contents measured by 613
EMPA and SIMS for both eruptions, different melt fraction vs T relationship and eruptive vent 614
location. Fondo Riccio’s explosive eruption occurred at centre of the CF caldera, while the more 615
“effusive-like” Minopoli 1 eruption occurred along a fissure fracture influenced by the regional 616
fault system in the northern portion of the CF caldera. 617
The timescale of evolution of Fondo Riccio magmatic system can be constrained based on rates 618
of heat loss from the nearby geothermal system of La Mofete, the volume of the system and the 619
difference between the enthalpy at the liquidus and the enthalpy at the lowest melt fraction (fm = 620
0.05). The results show that Fondo Riccio’s system has evolved over a time interval of 6.5 ± 3.5 ka, 621
meaning that the magma probably evolved from a basaltic trachy andesitic melt to a trachytic 622
composition over about 6000 years. Thermal timescale calculation for Minopoli 1 gives estimate of 623
potential evolving of the system from a basaltic to a trachy andesitic composition in a time span of 624
2.5 ± 1.5 ka. 625
626
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- Gaetani, G. A., and E. B. Watson, 2000 Open system behavior of olivine-hosted melt inclusions, 701
Earth Planet. Sci. Lett., 183, 27–41. 702
- Ghiorso M.S., Sack R.O. 1995 Chemical mass transfer in magmatic processes IV. A revised and 703
internally consistent thermodynamic model for the interpolation and extrapolation of liquid–solid 704
equilibria in magmatic systems at elevated temperatures and pressures. Contrib Mineral Petrol 705
119:197–212 706
- Kress, V. C. & Ghiorso, M. S. (2004). Thermodynamic modeling of post-entrapment 707
crystallization in igneous phases. Journal of Volcanology and Geothermal Research 137, 247-260. 708
- Lange, R. A. 1994. The effect of H2O and CO2 on the density and viscosity of silicate melts. In 709
Volatiles in Magmas. Reviews in Mineralogy 30, pp 331-370. 710
- Lowenstern, J.B., 1994.Dissolved volatile concentrations in ore-forming magma. Geology 22, 711
893–896. 712
- Lima A., De Vivo B., Spera F.J., Bodnar R.J., Milia A., Nunziata C., Belkin H.E., Cannatelli C., 713
(2009) Thermodynamic model for uplift and deflation episodes (bradyseism) associated with 714
magmatic-hydrothermal activity at the Campi Flegrei (Italy). Earth-Science Reviews 97, 44-58. 715
- Marianelli P., Sbrana A., Proto M. 2006 Magma chamber of the Campi Flegrei supervolcano at 716
the time of eruption of the Campanian Ignimbrite. Geology, v. 34, n. 11, p. 937–940. 717
- Metrich N., Wallace P.J. 2008 Volatiles abundance in basaltic magmas and their degassing paths 718
tracked by melt inclusions. Rev. Mineral Geochem 69, 363-402 719
- Michael P.J., McDonough W.F., Nielsen R.L. and Cornell W.C., 2002 Depleted melt inclusions in 720
MORB plagioclase: messages from the mantle or mirages from the magma chamber?. Chem. Geol. 721
183 (2002), pp. 43–61. 722
- Moore G. 2008 Interpreting H2O and CO2 contents in melt inclusions: constraints from solubility 723
experiments and modelling. In Minerals Inclusions and Volcanic Processes, Reviews in Mineralogy 724
69, pp. 333-361 725
- Moore G., Carmichael I.S.E. 1998 The hydrous phase equilibria (to 3kbar) of an andesite and 726
basaltic andesite from western Mexico: constraints on water content and conditions of phenocrysts 727
growth. Contrib. Mineral Petrol, 130, 304-319 728
- Morgan, G.B., London, D., 1996. Optimizing the electron microprobe analysis of hydrous alkali 729
aluminosilicate glasses. Am. Mineral. 81, 1176–1185. 730
- Ochs F.A., Lange R.A. 1999 The density of hydrous magmatic liquids. Science, 283, pp. 1314-731
1317 732
- Orsi G., de Vita S. and Di Vito M., 1996. The restless, resurgent Campi Flegrei nested caldera 733
(Italy): constraints on its evolution and configuration. J. Volcanol. Geotherm. Res., 74, 179-214 734
- Pappalardo L., Civetta L., D’Antonio M., Deino A.L., Di Vito M.A., Orsi G., Carandente A., de 735
Vita S., Isaia R., Piochi M., 1999. Chemical and isotopical evolution of the Phlegraean magmatic 736
system before the Campanian Ignimbrite (37 ka) and the Neapolitan Yellow Tuff (12 ka) eruptions. 737
J Volcanol Geotherm Res, 91, 141-166 738
- Pappalardo L., Piochi M., D’Antonio M., Civetta L., Petrini R., 2002. Evidence for multi-stage 739
magmatic evolution during the past 60 ka at Campi Flegrei (Italy) deduced from Sr, Nd and Pb 740
isotope data. J. Petrol., 43, 1415-1434 741
- Peccerillo A. 2005 Plio-Quaternary Volcanism in Italy. Springer, 365 p. 742
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Microbeam Analysis, v. 20, pp. 104-105. 744
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12. 644 pp. 747
- Roggensack K. 2001 Sizing up crystals and their melt inclusions: a new approach to crystallization 748
studies. Earth Planet Sci Lett, 187, pp 221-237. 749
- Rolandi G., Bellucci F., Heizler M.T., Belkin H.E. and De Vivo B., 2003. Tectonic controls on the 750
genesis of ignimbrites from the Campanian Volcanic Zone, southern Italy. Mineralogy and 751
Petrology, 79, 3 - 31. 752
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Longman/Wyllie. Harlow/New York 754
- Rosi, M., and Sbrana, A., 1987, Phlegraean Fields: Quaderni de “La Ricerca Scientifica”: 755
Consiglio Nazionale delle Ricerche Monograph 114, Volume 9, 175 p. 756
- Shimizu, N., Hart, S.R., 1982. Application of the ion probe to geochemistry and cosmochemistry. 757
Annu. Rev. Earth Planet. Sci. 10, 483–526. 758
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Petrology 4, 209–220. 760
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Ustinov, V.I., 1990. Volatile regime and crystallization conditions in Etna hawaiite lavas. Geochem. 762
Int. 53–65. 763
- Sparks R.S.J., Barclay J., Jaupart C., Mader H.M., Phillips J.C. 1994 Physical aspects of magmatic 764
degassing I. Experimental and theoretical constraints on vesciculation. Rev. Mineral 30, pp 413-446 765
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of bubble-bearing magmas; discussion and reply [modified]. Earth and Planetary Science Letters 767
175, 327-334. 768
- Wallace P.J., 2005 Volatiles in subduction zone magmas; concentrations and fluxes based on melt 769
inclusion and volcanic gas data. J. Volcanol. Geotherm. Res. 140, 1-3, 217-240 770
- Watson, E. B. (1976). Glass inclusions as samples of early magmatic liquid; determinative method 771
and application to South Atlantic basalt. Journal of Volcanology and Geothermal Research 1, 73-84. 772
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geochemistry of heterogeneous Mexican tin rhyolite magmas deduced from compositions of melt 774
inclusions. Geochim. Cosmochim. Acta 60, 3267–3283. 775
- Webster, J.D., Raia, F., De Vivo, B., Rolandi, G., 2001. The behavior of chlorine and sulfur 776
during differentiation of the Mt. Somma– Vesuvius magmatic system. Mineral. Petrol. 73, 177–201. 777
- White, S.M., J.A. Crisp, and F.J. Spera, 2006. Long-term volumetric eruption rates and magma 778
budgets, Geochemistry, Geophysics, Geosystems, 7, Q03010, doi:10.1029/2005GC001002. 779
- Wohletz, K., Civetta, L. & Orsi, G. (1999). Thermal evolution of the Phlegraean magmatic 780
system. Journal of Volcanology and Geothermal Research 91, 381-414. 781
- Zollo, A., Judenherc, S., Auger, E., D’Auria, L., Virieux, J., Capuano, P., Chiarabba, C., 782
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Campi Flegrei caldera from 3-D active seismic imaging: Geophysical Research Letters, v. 30, 2002, 784
doi: 10.1029/2003GL018173. 785
786
787
788
Figure Captions 789
Fig. 1 Schematic geological map of Campi Flegrei Volcanic District (CFVD), FR = Fondo Riccio, 790
Mi1= Minopoli 1. Modified after Orsi et al. (1996) and Peccerillo (2005). 791
792
Fig. 2 CaO-MgO-Al2O3 triangular diagrams (Watson, 1976) for melt inclusions hosted in olivine 793
(circles) and pyroxene (squares). Point “I” is the intersection of olivine and pyroxene fractionation 794
lines and represent the composition of the magmatic liquid at the time of melt inclusion formation. 795
(a) Fondo Riccio, (b) Minopoli 1. 796
797
Fig. 3 Fondo Riccio’s phase proportion diagrams as a function of temperature for MELTS 798
simulation at variable oxygen fugacity, P = 0.15 GPa and H2O = 3wt%. Ap = apatite, Bio = biotite, 799
Cpx =clinopyroxene, Ksp = alkali feldspar, Ol = olivine, Plag = plagioclase feldspar, Rh-ox = 800
rhombohedral oxide, Sp = spinel. (a) QFM+1, (b) QFM. 801
802
Fig.4 Oxides diagram for Fondo Riccio in the best cases of 0.15 GPa, 3wt% H2O and varying fO2 803
between QFM and QFM+1. (a) FeOtot, (b) Al2O3, (c) Na2O, (d) K2O, (e) SiO2 and (f) CaO . 804
805
Fig. 5 Minopoli 1 phase proportion as a function of temperature for MELTS simulation at variable 806
oxygen fugacity, P = 0.3 GPa and H2O = 2wt%. Ap = apatite, Bio = biotite, Cor = corundum, Cpx 807
=clinopyroxene, Ksp = alkali feldspar, Leu = leucite, Ol = olivine, Plag = plagioclase feldspar, Rut 808
= rutile, Sp = spinel. (a) QFM, (b) QFM+1. 809
810
Fig. 6 Oxides diagram for Minopoli 1 in the best case produced by MELTS, P = 0.3 GPa, 2wt% 811
H2O and fO2 = QFM. (a) SiO2, (b) CaO, (c) FeOtot, (d) Al2O3, (e) Na2O and (f) K2O. 812
813
Fig. 7 Variation of melt physical properties for Fondo Riccio along the liquid line of descent for the 814
case P= 0.15 GPa, 3wt% H2O and QFM+1. (a) Density of melt versus T, (b) Dissolved water 815
content versus T, (c) melt viscosity versus T, (d) volume fraction of water versus T. H2O saturates 816
at 1070°C. 817
818
Fig. 8 Variation of melt physical properties for Minopoli 1 along the liquid line of descent for the 819
case P= 0.3 GPa, 2wt% H2O and QFM. (a) Density of melt versus T, (b) Dissolved water content 820
versus T, (c) melt viscosity versus T, (d) volume fraction of water versus T. there is no saturation of 821
water along the liquid line of descent for this case. 822
823
Fig. 9 Timescale evolution and temporal crystallization history showed by phase proportion in 824
function of magma temperature. (a) Fondo Riccio, (b) Minopoli 1. According to the best fit model, 825
ages represent time before each eruption when specific phase mineral begin crystallizing. 826
827
828
829
830
831
832
833
834
835
836
837
838
839
Supplementary material Figure captions 840
Fig. A Total Alkali Silica diagram (Le Bas et al., 1986) showing the MELTS results simulations for 841
variable fO2. Symbols are shown in the legend. Shaded area represents field for all Fondo Riccio 842
melt inclusions data (Cannatelli et al., 2007). 843
844
Fig. B Phase proportion as a function of temperature for MELTS simulation at variable oxygen 845
fugacity, P = 0.2 GPa and H2O = 2wt% for Fondo Riccio. Ap = apatite, Bio = biotite, Cor = 846
corundum, Cpx =clinopyroxene, Ksp = alkali feldspar, Leu = leucite, Ol = olivine, Plag = 847
plagioclase feldspar, Rut = rutile, Sp = spinel. (a) QFM+2, (b) QFM+1, (c) QFM, (d) QFM-1, (e) 848
QFM-2. 849
850
Fig. C Minopoli 1 calculated compositions for fO2 = QFM+1, H2O = 2wt% and P = 0.2 GPa and 0.3 851
GPa. (a) Feldspar, (b) Spinel. 852
853
Fig. D Fondo Riccio’s calculated mineral compositions for the best case of MELTS simulations. (a) 854
Circles = olivine, diamonds = clinopyroxene; (b) triangles = feldspar; (c) triangles = spinel. Grey 855
symbols are MELTS generated data, open symbols are mineral data collected by EMPA. 856
857
Fig. E Minopoli 1’s calculated mineral compositions for the best case of MELTS simulations. (a) 858
Circles = olivine, diamonds = cpx; (b) triangles = feldspar. Grey symbols are MELTS generated 859
data, open symbols are mineral data collected by EMPA. 860
Recent Sediments
Volcanics younger than 15 ky
Volcanics older than 15 ky
Neapolitan Yellow Tuff (15 ky)
Campanian Ignimbrite (39ky) and volcanics between 39 and 15 ky
Caldera
Crater
Fault
0 4
km
MgO
CaO
5% M
gO
20% CaO
40% CaO
30 % Al2O3
I
Fondo Riccio
I
5% M
gO
40% CaO
20% CaO
30 % Al2O
3
0.66
0.39
1.030.72
0.90 1.12
1.15o2
o4 o1o6
p6
p11
p12
(a)
(b)CaO
10%
MgO
20% CaO
45 % CaO
30 % Al2O3
I
Minopoli 1
I
1.21
0.90
0.591.70
0.981.06
1.17
10%
MgO
45% CaO
20% CaO
30 % Al2O
3
Al2O3
MgOAl2O3
p2
p8p3 p6M1
p6M2
Fondo Riccio 3wt% H2O, 1.5 kbar
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Ol inCpx in
Sp in
Pl inK-fsp inBio inH2O saturates at
1070°C
fO2 = QFM+1
Ksp
T(°C)
f m (m
elt f
ract
ion)
(a)
1280 1180 1080 980 880 780
0
0.1
0.6
0.5
0.4
0.3
0.2
0.9
0.8
0.7
1
Ol inCpx in
Sp in
Ap in
Pl inK-fsp inBio inH2O saturates at
1096°C
fO2 = QFM
T(°C)
f m (m
elt f
ract
ion)
(b)
1280 1180 1080 980 880 780
10
12
14
16
18
20
22
24
26
0 2 4 6 8 10
Ol inMt inCpx in
Ap in
PlagK-fsp
Bio in
Al2O3
0
1
2
3
4
5
6
7
8
9
0 2 4 6 8 10
Ol in
Mt inCpx in
Ap in
PlagK-spar
Bio in
Fondo Riccio 1.5kbar, 3wt% H2O
FeO
MgO MgO
0
1
2
3
4
5
0 2 4 6 8 10
Ol in
Mg inCpx in
PlagK-spar
Na2O
MgO
0
1
2
3
4
5
6
7
8
9
10
0 2 4 6 8 10
Mag inCpx in
PlagK-spar
K2O
MgO
40
44
48
52
56
60
0 2 4 6 8 10
Ol in
Cpx in
PlagK-spar
Apa in
Bio in
Mag in
SiO2
MgO
0
2
4
6
8
10
12
0 2 4 6 8 10
Mg in
Cpx in
Ap inPlagK-spar
Bio in
Ol in
CaO
1.5 kbar - QFM+1 - 3wt% H2O1.5 kbar - QFM-1 - 3wt% H2O1.5 kbar - QFM - 3wt% H2O
Mi hosted in CpxMi hosted in Ol
MgO
Ol in
Bulk rock Fondo Riccio
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.01300 1200 1100 1000 900 800
T(°C)
Ol inCpx in
ap insp in
k-feld in
Pl in
H2O, bio in
1300 1200 1100 1000 900 800
0.0
0.1
0.3
0.4
0.5
0.2
0.6
0.7
0.9
1.0
0.8
bio in
Pl in
k-feld in
sp inap in
Cpx inOl in
fO2 = QFM
f m (m
elt f
ract
ion)
fO2 = QFM+1
f m (m
elt f
ract
ion)
Minopoli 1 2wt%H2O and 3kbars
T(°C)
H2O saturates at800°C
Bulk rock this study
40
42
44
46
48
50
52
54
56
58
60
0 2 4 6 8 10 12
SiO2
MgO
0
2
4
6
8
10
12
14
16
0 2 4 6 8 10 12
MgO
CaO
Minopoli 1, QFM+1, 3kb, 2wt% H2O
0
1
2
3
4
5
6
7
8
9
10
0 2 4 6 8 10 1210
12
14
16
18
20
22
24
26
0 2 4 6 8 10 12
FeOtot
MgO
Al2O3
MgO
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
0 2 4 6 8 10 120
1
2
3
4
5
6
7
8
9
0 2 4 6 8 10 12
Na2O
MgO MgO
K2O
Melt Inclusion in Olivine
Melt Inclusion in CPX
cpx in
cpx in
cpx in
cpx in
cpx incpx in
ap in
ap inap in
ap in
ap in
ap in
sp in
sp in
sp insp in
sp in
sp in
K-fds
K-fds
K-fds
K-fdsK-fds
Pl inbio in
0
2
4
6
8
10
12
7008009001000110012001300
Dis
solv
ed H
2O (w
t%)
T(°C)
2240
2260
2280
2300
2320
2340
2360
2380
2400
2420
Den
sity
(kg/
m-3
)
T(°C)
7008009001000110012001300
T(°C)
1300 1200 1100 1000 900 800 700
Vis
cosi
ty (P
a*s)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
8009001000110012001300
T(°C)
Volu
me
frac
tion
of H
2O
FONDO RICCIO
0.00E+00
5.00E+04
1.00E+05
1.50E+05
2.00E+05
2.50E+05
(a) (b)
(c) (d)
0
2
4
6
8
10
6007008009001000110012001300
T (°C )
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
6007008009001000110012001300
2200
2250
2300
2350
2400
2450
2500
6007008009001000110012001300
Den
sity
(kg/
m3 )
T (°C )
Vis
cosi
ty (P
a*s)
Dis
solv
ed w
ater
(wt%
)
T (°C )
Volu
me
frac
tion
wat
er
T (°C )
MINOPOLI 1
(a) (b)
(c) (d)
70080090010001100120013000.00E+00
1.00E+05
5.00E+05
4.00E+05
3.00E+05
2.00E+05
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.01300 1200 1100 1000 900 800
T(°C)
f m (m
elt f
ract
ion)
(b)
1280 1180 1080 980 880 780
0
0.1
0.6
0.5
0.4
0.3
0.2
0.9
0.8
0.7
1
T(°C)
f m (m
elt f
ract
ion)
(a) 0
1
2
3
4
5
6
7
8
910
1.0
2.0
Time (K
y)Tim
e (Ky)
0
3.0
4.0
4.1 ky3.2 ky
1.8 ky1.7 ky
1.5 ky1.6 ky
10.3 ky
8.2 ky
5.7 ky4.9 ky
4.8 ky
Sample Mi1-C1 o6 M1 FR-C1 o2 M1
SiO2 47.96 46.83TiO2 0.78 0.89Al2O3 14.61 12.77FeOtot 7.73 8.22MnO 0.08 0.18MgO 9.37 9.45CaO 8.94 9.42Na2O 1.57 1.33K2O 3.92 4.41P2O5 0.56 0.62NiO 0.05 n/aSO2 0.21 0.40F 0.17 0.14Cl 0.43 0.44H2O 3.45 4.48
Picro-basalt
BasaltBasalticandesite
AndesiteDacite
Rhyolite
Trachyte
TrachydaciteTrachy-andesite
Basaltictrachy-andesite
TephriteBasanite
Phono-
Tephri-phonolite
Phonolite
Foidite
Picro-basalt
Basalt BasalticandesiteAndesite
Dacite
Rhyolite
Trachyte
TrachydaciteTrachy-andesite
Basaltictrachy-andesiteTrachy-basalt
TephriteBasanite
Phono-Tephrite
Tephri-phonolite
Phonolite
Foidite
QFM, 3%H2O
Na2O+K2O
16
14
12
10
8
6
4
2
0 35 40 45 50 55 60 65 70 75
Trachy-basalt
SiO2
Tephrite
Na2O+K2O16
14
12
10
8
6
4
2
0 35 40 45 50 55 60 65 70 75
SiO2
1 kbar1.5 kbar2 kbar
3 kbar
FR MI
(a)
(b)QFM+1, 3%H2O
Bio in
Ap in
Sp in
CPX
QFM-10
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1300 1200 1100 1000 900 800 T(°C)
f m (m
elt f
ract
ion)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
QFM+1
Ol
CPXCPX
Sp in
Bio in
T(°C)
f m (m
elt f
ract
ion)
Leu in
1300 1200 1100 1000 900 800
T(°C)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
f m (m
elt f
ract
ion)
QFM-2
Ol
CPX
Sp in
1300 1200 1100 1000 900 800
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
T(°C)
f m (m
elt f
ract
ion)
FONDO RICCIO (2kb, 2wt% H2O)
Ap
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
T(°C)
f m (m
elt f
ract
ion)
QFM
QFM+2
Bio in
Rut+Leu in
H2O sat
Ap in
Bio in
Sp in
Ap in
Bio in
Cor+Leu in
H2O sat
H2O sat
H2O sat
H2O sat
Ol inSp in
Cpx in
Ap in
Plag, Ksp in
Ol in
Cpx in
Plag, Ksp in
(a) (b)
1300 1200 1100 1000 900 800
1300 1200 1100 1000 900 800
Ol inCpx in
Plag, Ksp in
Ol inCpx in
Plag, Ksp in
Ap in
Ol inCpx in
Plag, Ksp in
(c)
(e)(d)
albite sanidine
anorthite
MELTS data (2kb, 2%H2O, QFM+1) albite sanidine
anorthite
MELTS data (3kb, 2%H2O, QFM+1)
Minopoli 1 (2kb, 2wt% H2O)Minopoli 1 (EMP data)
(illmenite)
(ulvospinel)
(magnetite) (Hematite)FeO
TiO2
Fe3O4 Fe2O3
FeTiO3
Fe2TiO4
Fe2TiO5
Minopoli 1 (3kb, 2wt% H2O)Minopoli 1 (EMP data)
(illmenite)
(ulvospinel)
(magnetite) (Hematite)FeO
TiO2
Fe3O4 Fe2O3
FeTiO3
Fe2TiO4
Fe2TiO5
Minopoli 1 (EMP data)Minopoli 1 (EMP data)
Ab Or
An
En Fs
Wo50
Fo 84.5 87.0
(a) (b)
FeTiO3
Fe2TiO4
(illmenite)
(ulvospinel)
Fe3O4(magnetite)
Fe2TiO5
(Hematite)FeO Fe2O3
TiO2(c)
MELTS generated dataObserved data
Minopoli, QFM-1, 3kb, 2wt% H2O
Wo = 50
En Fs
albite sanidine
anorthite
Fo78
MELTS generated dataObserved data
FeTiO3
Fe2TiO4
(illmenite)
(ulvospinel)
Fe3O4(magnetite)
Fe2TiO5
(Hematite)FeO Fe2O3
TiO2
(a) (b)
(c)
